336 TEXT-BOOK OF PHYSIOLOGY. 



certain number of millimeters, yet retains a fairly constant general average, 

 the result of an adjustment between the number of heart-beats per minute 

 and the amount of the resistance offered to the escape of blood into the capil- 

 laries and veins. Though the tracing fails to record accurately the diastolic 

 and systolic pressures it approximates a certain average or mean of the pres- 

 sure thus recorded, which represents the power driving the blood through 

 the vessels. It is frequently stated that in a tracing in which the respiratory 

 undulations are absent, the mean pressure is the arithmetic mean of 

 the systolic and diastolic pressures. This is, however, not strictly correct, 

 for if the pressure is recorded by means of a spring manometer or a sphygmo- 

 graph applied over the artery of man a record much different in appearance 

 and similar to that shown in Fig. 157 will be obtained. In such a record 

 it will be observed that the return of the pressure from the systolic to the 

 diastolic level not only occupies a longer time than the passage from the 

 diastolic to the systolic level, but that the line of descent is interrupted by a 

 secondary rise and fall of pressure before the original diastolic level is 

 reached. It is evident, therefore, that the pressure is low for a longer period 

 than it is high and hence the mean pressure cannot be the arithmetic mean 

 between the diastolic and systolic pressures. The mean pressure, however, 

 can for a given period at least be experimentally determined. Thus, if at 

 some one point between the artery and the manometer, the lumen of the 

 connecting tube be largely obliterated by a constriction, the variations in 

 the pressure following the systole and diastole of the heart will be largely, if 

 not entirely excluded, and the mercury, instead of rising rapidly in the man- 

 ometer and fluctuating with each heart-beat, will rise slowly to a certain level 

 and then remain at rest. The number of millimeters of mercury thus 

 supported represents the mean or absolute pressure. The same result can be 

 obtained by employing the compensatory manometer of Marey which 

 presents a constriction of this character. From many experiments made by 

 Dawson it has been learned that the mean pressure lies nearer to the diastolic 

 than to the systolic pressure and may be expressed numerically by the state- 

 ment that it is equal in millimeters of mercury to the diastolic pressure plus 

 one-third of the pulse pressure. In a tracing in which the respiratory undu- 

 lations are present the mean pressure can be calculated. The method by 

 which this is done, however, is rather complicated and need not be detailed 

 here. In a general way the mean pressure in such a tracing may be repre- 

 sented by a line drawn horizontally across the tracing midway between the 

 apex and trough of the undulation. 



Estimates of the Mean Arterial Pressure. Because of the difficulty 

 in obtaining the pressure in small arteries, the experimental determinations 

 have for the most part been confined to large arteries such as the carotid, 

 brachial, and femoral, and hence the results which have been obtained have 

 reference to the lateral pressure in the aorta or in the large vessels which 

 immediately arise from it. The pressure obtained in the usual way at the 

 central end of a divided carotid is generally known as the "end pressure" 

 and represents the mean lateral pressure in the aorta or in the innominate 

 artery. Among the results thus obtained in different experiments from the 

 carotid artery of different animals are the following: In the horse, from 

 122 to 214 mm. Hg.; in the dog, from 140 to 160 mm.; in the cat, 150 mm.; 



